--resume(-r)
Resumes the most recent merge list that has been aborted due to an error. This re-uses the options that were given with the original command that's being resumed, and the user may also provide additional options when calling --resume. Please note that this operation will only return an error on failure. If there is nothing for portage to do, then portage will exit with a message and a success condition. A resume list will persist until it has been completed in entirety or until another aborted merge list replaces it. The resume history is capable of storing two merge lists. After one resume list completes, it is possible to invoke --resume once again in order to resume an older list.

It looks like it is possible. I am thinking maybe the --pretend command initiates and terminates the emerge itself. It would be either necessary to:
1. copy the logic from the --resume into the --pretend functionality.
2. create a autonomous --resume -p functionality.